I think the only people, surprised by the success of the Caucasus mountains guys, are the ones who are not familiar with the grappling styles they have trained, since childhood.
They (its the same in my native Bulgaria) start with freestyle wrestling, practiced in countryside environment (meaning, while herding sheeps, or doing farm work, similar to Iowa's wrestlers).
The most successful flow into the local government sports gym.
Now, here is the difference:
Point 1:
They get professional coaching advises by a person who was successful in the sport AND got a national sports academy education in coaching, in his respective style.
Point 2:
Those gyms are usually registered with the national Judo, Sambo and Wrestling federations, meaning they compete in all 3.
Eventually, the natural selection will direct some in one specific sport, or they will end up using their skills in a sport like MMA.
And now we see those same guys, upgrading their already existing game (in terms of grappling, great takedowns, followed by heavy top pressure and pins), to the modern aspects of MMA.
As everyone in the sport (MMA) has said, MMA grappling is quite different from pure grappling.
I think its strange to have this conversation in 2021, but its widely known, that a guy who started grappling as a child, will most probably be dominating MMA, if he follows the path, win some medals and is constantly upgrading his game.
I dont see why we are talking about BJJ as a separated entity of MIXED Martial Arts.
Its just one of the aspects.
And it really depends on many factors.
Here is one:
Jiujitsu is played on open space area, with restarts, if the contestants leave the area.
MMA is played in closed area, without breaks (almost).
The area is 3D, meaning one can be pinned and his game neutralised in VERTICAL position.
BJJ has no such gameplay.